Human cancers develop in a complex environment composed of blood vessels, fibroblasts, and immune cells. The tumor microenvironment contributes to cancer development, progression, and response to therapy. Nearly two-thirds of all cancer patients receive radiation therapy during their illness. Although greater than 75% of these patients are treated with the intent to cure, it remains unclear if radiation cures tumors by killing tumor parenchymal cells or supporting stromal cells such as endothelial cells. The long-term goal of this study is to determine the mechanisms of tumor cure by radiation to enhance the efficacy of radiation therapy in the clinic. The overall objective of this application is to deine the cellular target(s) of radiation therapy in primary mouse models of cancer. Utilizing the two highly efficient site-specific recombinases, Cre and Flp, it is possible to contemporaneously mutate different genes specifically in tumor parenchymal cells and stromal cells. This dual recombinase technology enables the radiosensitivity of either the tumor parenchyma or the tumor microenvironment to be manipulated in primary tumors by mutating genes involved in DNA damage repair and apoptosis. The central hypothesis of this study is that radiation therapy cures tumors by killing tumor parenchymal cells rather than endothelial cells. Using micro-CT targeted irradiation and genetically engineered mouse models of primary lung adenocarcinoma and soft tissue sarcoma, the following specific aims will be completed: Aim 1: Determine the contribution of endothelial cells to tumor response to radiotherapy. Aim 2: Determine the contribution of tumor parenchymal cells to tumor response to radiotherapy. By selectively sensitizing or protecting either tumor parenchymal cells or endothelial cells, the critical target() of radiation therapy in primary cancers will be defined. Evaluating the contribution of the tumor microenvironment to tumor response to radiation therapy will help determine the viability of targeting stromal cells to improve the efficacy of radiation therapy in the clinic.